It has been common practice to apply a resin coating to the surface of the seat belt webbing after weaving and dyeing. This resin coating is intended to permit the seat belt to be pulled out and retracted smoothly (or to improve retractability). On the other hand, yarns for seat belts are given a variety of treatments such as spinning finish in the spinning and drawing stages.
A known example of the coating resin for improvement of wear resistance of seat belts is a resin based on a urethane prepolymer (block copolymer). (Japanese Patent Publication No. 66948/1992) After application to the seat belt webbing, this coating resin undergoes heat treatment for crosslinking. The crosslinked resin imparts very good slip properties initially and retains them at a certain level even after use for a long period of time.
A known example of the conventional treatment for seat belt yarns is a compound composed mainly of a branched alcohol, an ester of higher fatty acid, and a nonionic surface active agent (not containing propylene oxide), as disclosed in Japanese Patent Laid-open No. 175966/1990. The first two components function as a smoothing agent and the last component contributes to light resistance.
Unfortunately, the resin-coated seat belt, especially the one coated with a resin of crosslinking type, has the disadvantage of its surface coating becoming stiff after use for a long period of time. The stiff surface coating resin gradually drops off due to repeated rubbing against the nylon belt guide. In addition, the belt surface becomes soiled with time. Stiffening and soiling prevent the seat belt from being pulled out and retracted smoothly.
In the case where seat belts are made of spin-dyed yarns, the step of dyeing the seat belt webbing is not necessary. Therefore, attempts have been made to perform resin coating (for friction reduction) in the yarn-making step, thereby omitting not only dyeing but also resin coating after the seat belt webbing has been made. However, the conventional coating resin for friction reduction is so sticky that it prevents smooth unwinding from cheese package if it is given to yarns. In addition, the coating resin applied to yarns give rise to scum at the belt-weaving stage. Therefore, such attempts are not successful so far.
The seat belt made of resin-coated yarns may be used as such without subsequent resin coating. The seat belt of this type does not decrease rapidly in slipperiness due to the dropping of resin. However, it does not fully decrease in fiber-to-fiber friction or fiber-to-metal friction. Hence it is so poor in initial slipperiness and wear resistance that it is not yet in practical use.
The present invention was completed to address the above-mentioned problems involved in the conventional technology. It is an object of the present invention to provide a friction-reducing treatment which can be applied to both seat belt yarns and seat belt webbing. This friction-reducing treatment contributes to the low friction coefficient of yarns constituting the seat belt and hence makes the seat belt highly slippery in the initial stage. In addition it retains its good slip characteristics for a long period of use. The friction-reducing treatment realizes the seat belt which keeps its good retractability and wear resistance for a long period of use.
It is another object of the present invention to provide a treatment for reducing friction of seat belts which is suitably used in the case where dyeing and resin finishing are not performed on the seat belt webbing.